Fiber lasers are used for producing high average power, high beam quality, high beam stability, and highly efficient sources of laser radiation. Production of light at 1, 1.5, and 2 μm can be performed using Yb, Er/Yb, and Tm:silica fibers, respectively. Other wavelengths can be obtained using Raman lasers (generally up to 2 μm in continuous wave operation) or by wavelength conversion using bulk crystals. Such wavelength conversion processes include harmonic, sum, and difference frequency generation. Harmonic generation is used to produce shorter wavelengths at multiples of the fundamental laser frequency. An example of harmonic generation is second harmonic generation (SHG) for producing green light (532 nm) from a 1064 nm Yb:silica fiber laser pump. Longer wavelengths are typically produced using difference frequency generation (DFG). More commonly, DFG is performed in a cavity to enhance the efficiency, i.e. in an optical parametric oscillator (OPO). OPOs are generally used to generate longer wavelengths from a shorter wavelength and can produce a broad range of wavelengths which are longer than the fiber laser pump source.
Multiple applications require high average power and high beam quality sources in the mid-infrared. In addition, such applications may also require modulation of output power or temporal characteristics. These applications include material processing (especially of glasses and plastics), spectroscopy of materials with mid-infrared signatures, remote sensing through the atmosphere, laser radar, free-space laser communications, medical applications, infrared countermeasures, and others.
These applications often require the ability to quickly modulate the output radiation. For example, in a material processing cutting application, the OPO output radiation needs to be turned on and off with short transition times and stability of output power. The OPO light can be directly modulated by external modulators, such as acousto-optic or electro-optic modulators. However, modulators in the mid-IR are not commonly available with low losses and fast transition times. The modulator itself adds to the complexity and cost of the system. In addition, if the power must be modulated externally to the laser generator, then there can be a large amount of wasted power, both optical and electrical, as the beam is continuously generated but not used.
There is thus a need for a system and method for modulation of OPO output without the use of external modulators and that can be done in the mid-IR range.